Redundant actuators increase spindle dexterity

Seoul, Korea-As product life cycles shrink and part geometry gets more complex, manufacturing flexibility is a must to meet the demand for customer-oriented production and smaller lot sizes. By consolidating milling, drilling, and turning operations, modern machining centers offer flexibility. But because these machines typically use a spindle that is fixed in either a vertical or horizontal orientation, it's necessary to change workpiece position, or setup, to cut all five sides of a part. In an effort to decrease non-productive time such as tool changes, engineers design for higher speeds and accelerations. But once the chips actually start flying, higher traverse rates have little impact on overall cycle time.

To give manufacturers an extra edge when it comes to turn-around time from order to delivery, Seoul, Korea-based Sena Technologies presents a simple yet revolutionary new concept in universal machining called Eclipse. Introduced at EMO Paris '99, an exhibition devoted entirely to machine tools and machine components, engineers designed Eclipse with rapid manufacturing, rather than faster traverse rates, in mind. Eclipse uses a new parallel machine architecture that allows its spindle to tilt from full vertical to full horizontal, and sweep 360 degrees around the workpiece. Such spindle dexterity allows five-sided machining without chucking, unchucking, loading, unloading, and reloading the workpiece. "Eclipse minimizes total machining time by eliminating workpiece handling," says Sena Technologies President Tae Kim. "Extreme spindle dexterity and a large workspace allow Eclipse to drill, mill, and turn to create almost any shape part with a single setup."

The lightweight and high stiffness of parallel mechanisms (such as the Stewart-Gough platform) has attracted manufacturer and machine tool builder interest for some time. Giddings & Lewis, Ingersoll, Geodetics, and Hitachi Seiki have all applied such parallel mechanisms as mechanical platforms on commercial CNC machining centers, and more companies appear likely to follow suit. However, because of workspace limitations and a maximum spindle-tilt angle from vertical posture of 30 degrees or less, none of these machines so far satisfy prerequisites for rapid machining, according to Kim. He also notes the workspace is limited in most designs. To achieve rapid manufacturing, he explains, the spindle must tilt 90 degrees. Eclipse uses an "over-actuated" parallel mechanism to attain such motion. The machine exploits all the inherent advantages of parallel kinematic structures, while simultaneously expanding available workspace, and boosting spindle dexterity compared to existing designs.

Eclipse consists of three PPRS serial subchains that move independently on a fixed circular guide. P, R, and S denote prismatic, revolute, and spherical joints, respectively. The mechanism has six kinematic degrees of freedom (DOF), and eight actuated joints: three P joints along the circular guide, three P joints on the vertical columns, and two R joints on two of the vertical columns.

Stymied by singularities. In the early prototype design stage, engineers expected six servomotors would suffice for the six DOF machine. One servomotor drives each of the three vertical columns on the circular guide, and another three servomotors each drive one of three columns that feed the carriage on the vertical guideway. However, says Kim, the six-servomotor prototype revealed singularities within the mechanism that made 90-degree tilting impossible. Singularities for parallel mechanisms come in two types: end-effector and actuator singularities. In the former case, the tool of the mechanism loses one or more DOF, while in the latter case the tool can be said to gain one or more DOF.

The relative movements around the circumference of each vertical column determine the spindle’s tilt angle. When columns are spaced apart, the spindle maintains a vertical stance. As the columns move along the circular guide into a side-by-side position, the spindle proceeds into a horizontal posture.

Adding two actuators to the design averted two actuator singularities at approximately 30- and 60-degree spindle tilt angles. Installed at each of two pin joints of the carriages, one on the lower vertical column and the other on one of the two upper vertical columns, the extra actuators remove the singularities. These additional actuators produce an over-actuated prototype where the additional actuator exerts a reaction force that guides the spindle platform to the desired path at the actuator singularity position. As far as the motion control is concerned, the basic strategy is master-slave control to cover the actuator redundancy. "The two additional feed motors are controlled as a slave," explains Kim, "and the six feed motors are controlled as a master according to the inverse kinematic solution to realize tool tip movement for following cutter position motion in Cartesian space."

Unlike the symmetric designs typical of most Stewart-Gough Platform-type architectures, one fixed-length rod in one Eclipse prototype subchain is positioned under the spindle platform. This asymmetrical design ensures that the fixed rods don't interfere with the spindle motor that protrudes from the spindle platform opposite the spindle. To this effect, the spindle platform maintains vertical posture when the three vertical columns are spaced apart. And as the vertical columns move along the circular guide to a side-by-side position, the spindle platform goes into the horizontal posture with its tilting angle reaching 90 degrees.

Since the design objective for this over-actuated Eclipse prototype was to verify the performance of the new parallel mechanism, stiffness wasn't a main issue. When in a vertical position, the prototype achieved a stiffness of 1.2 N/µ and 0.6 N/µ along the spindle axis and radial direction, respectively. In a horizontal posture, stiffness was 1.5 N/µ and 3.0 N/µ along the spindle axis and radial direction, respectively. "With this stiffness we could only machine plastic samples," says Kim.

Once this mechanism was verified, however, Kim's team put engineering tools such as MATLAB for kinematic and dynamic simulation, I-DEAS for stiffness analysis of the mechanism, and OpenGL for graphic animation to visualize the motion of the mechanism, to work to build a real machine. When compared to the prototype, the Eclipse machine differs on three key differences:

All three vertical columns are directed upward from the base ring and identical with each other. In contrast, the prototype has one vertical column downward from the base ring.

Universal joints are substituted for the ball-socket joints in the prototype to improve stiffness.

The machine controller, an open-architecture PC-based CNC, is specially developed for the Eclipse machine.

The end result gives a calculated stiffness of at least 50 N/µ in any posture. The basic diameter of the circular guide is 2,000 mm, workspace is Ř300' 200, and maximum feed rate at the tool tip is 10 m/min.

By realizing both five-face machining and simultaneous five-axis machining within a single machine tool, the Eclipse mechanism fulfills all of Kim's multi-process machine requirements for rapid manufacturing. Adding two actuators to the two R joints eliminates singularity problems such that Eclipse serves as a platform for a rapid machining system that may significantly reduce machining lead time by eliminating unnecessary workpiece set-up time.

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